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Full text of "Mathematical And Physical Papers - Iii"

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which a mere trace was accidentally present without my knowledge.
160.   The green communicated to microcosmic salt by uranium after exposure to the reducing flame has a very peculiar corn-position, by means  of which the presence of uranium may be instantly  detected.    For  this  purpose  it is   sufficient to view through a prism the inverted image of the flame of a candle formed  by the bead,  the latter being  so  held  as  to  be  seen projected on a dark object.    The observation is perfectly simple, and occupies  only  a  few  seconds.    The  spectrum   exhibits an isolated band at the red extremity, followed by a very intense dark band of absorption.    A similar dark band, but not quite so intense, occurs in the green: beyond the green there is usually bufc little light seen.    As the absorption progresses the first dark band invades all the space from the red to the green, and the spectrum consists  of an  isolated   red  band   and  a  green  band divided into two.    In its mode of absorption, the medium has a strong general  resemblance to chlorophyll.    The   green  due to copper or to chromium shows nothing remarkable when viewed through a prism, and could not possibly be confounded with the green due to protoxide of uranium.    The absorption bands due to this oxide are riot completely brought out  till   the  bead is cold.
161.    Uranium produces the same effects with borax as with microcosmic salt, but they are less distinct, or at least easily produced.
162.    When the uranium contained in a bead of microcosmic salt is thoroughly oxidized, and the bead is gently heated, so as just to be self-luminous, the light which it gives out is not red, like that of most substances at a low heat, but green, or rather greenish white.
163.    Solutions of protoxide of uranium have a very remarkable effect on the spectrum, resembling more or less that of a bead of microcosmic salt coloured green by uranium.    Of course the absorption can be observed much better by means of a solution than by a mere bead.    I have  observed several bands of absorption in such solutions, but the cases which I have hitherto